1. Field of the Invention
The present invention relates to a UV light sensing element.
2. Description of the Related Art
In recent years the amount of UV light is increasing on the ground level due to the destruction of the ozone layer, which is a problem concerning the Earth""s environment. The increase in UV light shorter than 330 nm is particularly eminent on the ground level due to the decrease of ozone in the ozone layer.
Such an increase causes a serious influence on the health of mankind exemplified by generation of skin cancers, increase of hyper-sensitiveness to light due to DNA damages, and accelerated aging by light. UV light having a wavelength shorter than 320 nm has very high energy, and is called UV-B. It easily destructs DNA and causes various disorders in the human skin. UV light having a wavelength longer than 320 nm is called UV-A, and induces spots, freckles, and wrinkles in the human skin. It is not desirable from a cosmetic viewpoint. Therefore, it is necessary to measure UV light in a very wide range.
The measurement of UV light has been generally conducted with a UV light sensor employing an ordinary UV light sensing element coupled with a band pass filter. The band pass filter controls the coefficient of light transmission ranging from UV light to visible light using multiple reflection based on multiply layered thin films. An ordinary band pass filter having a transmission area in a wavelength range shorter than 400 nm, which includes a part of UV light, is accompanied with another transmission area for secondary light from 400 nm to 800 nm. Therefore, duplicated use of the band pass filter with another band pass filter cutting visible light is necessary for selective detection of light in the UV range. In reality, however, a band pass filter that cuts the visible region cannot transmit UV light of the shorter wavelength region. Therefore, selective transmission of UV light by coupling of band pass filters only has been difficult. Even if it is implemented using a combination of multiple band pass filters, the constitution of UV light sensing element inevitably becomes too complex and large-sized.
The conventional UV light sensing element based on such band pass filters has a difficulty in accurate determination of UV light since it exhibits a large fluctuation in the transmission wavelength region depending on the incident angle of UV light. In addition, the conventional UV light sensing element has shown a high dependency on an incident angle of UV light (deviation of UV light intensity from the cosine rule for the incident angle against the light-receiving surface). For this reason, installment of a waveguide on the sensing surface is necessary so that the incident UV light can enter almost vertically into the light-receiving surface. However, this brings about a defect that the size of light sensing unit in the UV light sensing element becomes inevitably large. Further, the above method is associated with inaccurate determination of incident light that enters the light-receiving surface with a low angle. Therefore, the accuracy of the measurement deteriorates for light containing a large portion of scattered components such as sunlight.
When a UV light detective sensor is to be made using an ordinary UV light sensing element, installation of band pass filters and waveguides is necessary in order to detect UV light of necessary wavelength and diminish the dependency on the incident angle. As a result, such UV light detective sensor has a complex and large-sized composition accompanied with a high production cost. In addition, its light sensitivity is inferior since the incident light has to pass through band pass filters and waveguides beforehand. Furthermore, the UV light sensing element exhibits rather quick deterioration due to the aging of employed band pass filers by UV light.
Namely, the target of the present invention is to provide a UV light sensing element that may be assembled to a UV light detective sensor, which is able to detect UV light of desired wavelength stably without using a band pass filter or a waveguide, and has a simple constitution, small size, stable and superior accuracy, and high sensitivity to light with low dependency on an incident angle at a low cost of production.
The present invention provides a UV light sensing element having a first electrode and a sensor, wherein the first electrode includes a semiconductor containing at least one element selected from Al, Ga and In with nitrogen or oxygen, and the sensor includes a semiconductor containing at least one element selected from Al, Ga and In with nitrogen, and a longer wavelength end of an absorption spectrum for the first electrode is located nearer to a shorter wavelength side than a longer wavelength end of an absorption spectrum for the sensor.
The sensor may be a layer, preferably a thin film layer and the first electrode may also be a layer, preferably a thin film layer.
The sensor layer may contain at least one element selected from C, Si, Ge and Pb.
Alternatively, the sensor layer may contain at least one element selected from Be, Mg and Ca.
The above sensor layer may also contain hydrogen.
Further, the present invention presents a UV light sensing element having at least the first electrode layer and the sensor layer provided on a substrate in this order.
Further, the above UV light sensing element may contain a second electrode layer.
In the UV light sensing element, one surface of the first electrode layer not adjacent to the sensor layer or one surface of the substrate not adjacent to the first electrode layer may be used as a light-receiving surface.
In the UV light sensing element, alternatively, one surface of the sensor layer not adjacent to the first electrode layer or one surface of the second electrode layer not adjacent to the sensor layer may be used as a light-receiving surface.